Comets

Abstract

Abstract Comets are small icy/dusty bodies left over from the formation of the solar system and represent the least altered bodies from the epoch of formation. They formed in the outer parts of our planetary system; some still reside in their primordial reservoir known as the Kuiper belt; those formed in the Uranus‐Neptune region were ejected to a spherical reservoir known as the Oort cloud; those formed near Jupiter and Saturn were ejected from our solar system entirely. The nuclei of comets are small (typically a few km) bodies composed of ices and dust. Of the ices 80% is water ice. The dust is composed of organics and silicates. As the nucleus is heated, the ices sublime and flow outward from the nucleus. The dust/gas mixture forms a large coma and even larger tails. To date, several spacecraft have visited comets and have made measurements in situ , but we have yet to retrieve samples and return them for study in laboratories. The vast majority of our knowledge of cometary composition comes, however, from remote observations obtained with ground‐based or Earth‐orbital telescopes. These data are gathered using spectroscopic and photometric techniques on a wide variety of comets. The spectrum of a comet is characterized predominantly by molecular emission coupled with an underlying continuum of sunlight reflecting from the dust. Every single day, about 100 tons of interplanetary material hits the upper atmosphere of Earth, and some of that material reaches the surface. A large percentage of the smallest particles originates in comets as dust grains that are released when ices sublime. These small cometary particles represent no hazard to Earth. Many of them are fluffy aggregates that “float” in the upper stratosphere for long periods of time and are occasionally collected for study by specially equipped high‐flying aircraft. Much can be learned about the nature of comets via observations from the ground and Earth orbit. However, one can never see the nucleus of the comet in this way, and information is inferred about its nature by using models. To study the nuclei of comets, it is necessary to send spacecraft to them. These spacecraft missions can range from simple flybys, to rendezvous missions, to landings and sample returns. The completed and planned cometary missions of the U.S. and other nations are described.